Sludge as a term is used herein is defined in common dictionaries. However, as a background to the present invention, an understanding of how sludge is formed is essential to the process of hydrocarbon tank sludge removal and tank cleaning.
Sludge Formation
Hydrocarbon based oils used in all sectors of the petroleum and petrochemical industry are often stored in tanks. Such storage occurs in crude oil and gas production, refineries, petrochemical plants, bulk plants, and oil storage terminals. Typical petroleum storage tanks will have a diameter from 100 to 400 feet and heights of 20 to 50 feet or more.
Over time, “sludge” forms in the bottom of these tanks. Sludge is a mixture of deposits, with a composition which varies from tank to tank. The composition of the sludge will depend upon the composition of the oil or oils that have been stored in a particular tank and/or the refining or petrochemical process associated with the tank.
A variety of materials contribute to sludge. In general, sludge can be formed from (for example) various combinations or proportions of naturally occurring sediments, higher molecular weight hydrocarbons, entrained water, as well as rust scales from piping and tank walls, inorganic debris (some from surface coatings, other from internal equipment and sampling operations), and process solids. Sludge is formed when these components are separated by gravity from the volume of liquid hydrocarbons in the storage tank. This multitude of combinations form a wide variety of sludge types, consisting of inorganic and organic materials that include, but are not limited to, organic resins, asphaltenes, paraffin compounds, heavy hydrocarbons, light hydrocarbons, gels, emulsions, rust particles, rust scales, mineral sediments, refining or petrochemical process solids, catalyst fines, pyrophoric iron sulfide deposits, glass bottles, soft lines, coating particles, coating scales, rags, gloves, cloth straps, plastics, styrene strings, bolts, iron pipe fittings, iron pipe connections, rocks, gravel, hard lines, tools, and metal straps.
Over time, the heavier elements in the stored oil will continue to migrate to the bottom of the tank and enter the sludge. As these heavier components concentrate, the sludge becomes more viscous, loses its flow characteristics, and (depending upon its composition) may even solidify. Since a large storage tank can hold a million barrels or more, and the volume which passes through a tank in the years between cleanings can be a large multiple of the tank volume, the storage tank can accumulate sludge from an enormous volume of oil.
Sludge Removal
Sludge removal or tank cleaning is required when sludge buildup interferes with or reduces the efficiency of the storage tank operation. Sludge removal or tank cleaning may also be required prior to the performance of a tank maintenance procedure, repair, modification or inspection.
All conventional techniques used to remove tank bottoms sludge from hydrocarbon storage tanks, while richly varied, can be classified under just two general methods—“Sludge Fluidization/Removal Method” and “Sludge Excavation/Removal Method”. The two Methods are similar in their need to overcome the wide array of physical and chemical characteristics associated with tank bottoms sludge that make it difficult to remove, such as high surface tension, agglomerated or solidified organic fractions, high organic and inorganic solids, and poor or nonexistent flow characteristics.
Conversely, the two Methods are distinctively different in the means by which removal of sludge from the tank is accomplished.
Sludge Fluidization
The primary method used for the removal of sludge is the Sludge Fluidization/Removal Method. In general, this method relies on the use of a liquid to fluidize the sludge for removal. The most common conventional iteration of this method is known as the “Cutter Stock” technique. The Cutter Stock technique is based on the use of a large quantity of low viscosity hydrocarbon liquid, heated or at ambient temperature, to mix into the sludge, reduce sludge viscosity, modify surface tension and thereby disperse the sludge throughout the carrier fluid to effect removal. In general, this method relies on large quantities of heated or ambient temperature diluents or cutter stock (various types of light oils such as diesel oil, light cycle oil, or light crude oil) being added into the tank and to the sludge at a ratio of cutter stock to sludge ranging from 1:1 up to 20:1 depending on tank bottom conditions and the specific iteration of the cutter stock method used. The heated or ambient temperature cutter stock is used as a carrier fluid to partially solubilize the organic fraction of the sludge while reducing the viscosity of the sludge through temperature and volumetric fluid dilution. The inefficiencies of cutter stock as a carrier fluid for sludge are partially offset by the high volume ratio of cutter stock to sludge. The mechanically dispersed sludge in the high volume of cutter stock is then subsequently removed via conventional pumping methods.
Sludge removal typically involves the delivery of cutter stock to the sludge by use of a centrifugal pump and through a fixed lance or nozzle, a manually articulated lance or nozzle, or a robotic device inside the tank shell in order to disperse the sludge throughout the cutter stock through circulation of the cutter stock and dispersed sludge followed by stripping (pump off) by centrifugal or sludge pumps until suction is lost. The ratio of cutter stock to sludge ranges from 1:1 to 20:1 (cutter stock at 1.0 to 20.0 times the volume of the sludge).
After these circulation operations have gone as far as they can, a substantial amount of organic solids (resins, asphaltenes) and inorganic solids (rust scale, surface coatings, mineral sediments) and debris will typically remain in the tank. These types of solids cannot be easily removed by the cutter stock method alone. The sludge solids remaining after the completion of this step is considered residual sludge.
Residual sludge is similarly dispersed and removed by further manual or robotic injection of heated or ambient temperature cutter stock and/or diesel or light cycle oil inside the tank. Residual sludge is manually pushed to sludge pumps positioned inside the tank and/or at the sump. Residual sludge that contains rust scale or other large debris must be manually removed by shovels or manually/mechanically removed by Air Vacuum trucks into Vacuum boxes for removal and disposal by the facility.
Floor and wall cleaning is generally accomplished by use of diesel or other light cycle oil with manual scrubbing to remove sticky sludge attached to these surfaces. Scrapers may be required. Filters will be required for rust scale and other debris.
Deoiling of the interior surfaces of the tank can be done by use of a soap injection pump and manual scrubbing followed by a wash with a high pressure fire hose. Wash water can be pumped by sludge pumps to the facility's container or line for disposal or treatment. Filters or other separation devices may be required for rust scale and other debris. The floor may then be detailed by squeegee and rags as required to remove visible oil and oily stains from tank surfaces.
The problems associated with the Sludge Fluidization/Removal Method in general and the Cutter Stock technique in particular, include:                Inefficient and time consuming (up to 3 Months for 300 foot diameter Tank)        Adds substantial volume, treatment time, cost and logistical transfer problems.        Heat Transfer is inefficient. Heat loss results in re-solidification of sludge and creates pumping, circulation and solids separation difficulties        Addition of cutter stock impacts physical and chemical characteristics of recovered crude oil, fuel oil, slurry oil, etc        Process safety concerns due to increased flammability and organic emissions.        Results in high volume of cutter stock that requires further processing or re-refining to remove dispersed sludge.Sludge Excavation        
The secondary method conventionally used for the removal of sludge is the Sludge Excavation/Removal Method. In general, this method relies on the use of manual or mechanical methods to physically collect, excavate, and remove the sludge from the tank in its existing condition. This method is time consuming, labor intensive and expensive. The personnel working within the tank are exposed to potential health risks as well as possible injury. Despite these drawbacks, manual removal is the only conventional removal mechanism for some types of tank bottom sludge conditions. Even when the previously discussed conventional methods are employed, the sludge is often not rendered sufficiently fluid by conventional methodology to be pumped out of the tank and at least some portion must be manually removed.
The problems associated with the Sludge Excavation/Removal Method include:                Inefficient, time consuming (up to 6 Months for 300 foot diameter Tank) and expensive;        Increases process safety concerns due to the requirement for working for extended periods of time in a confined space;        Results in high volume of waste requiring disposal.        
All of the previously discussed conventional sludge removal methods share a common shortcoming: a substantial decrease in storage tank utilization rates due to the inability of conventional methods to predictably complete tank cleaning operations and return the capital asset (storage tank) to service in a repeatable, efficient and cost effective manner.